Method and apparatus for correction of microbolometer output

ABSTRACT

A method and apparatus for correction of temperature-induced variations in the analog output characteristics of a microbolometer detector in an infrared detecting focal plane array utilizing electronic means to correct for the temperature variation of the individual microbolometer detector. The electronic circuitry and associated software necessary for implementation is also described.

This application claims benefit of our provisional application Ser. No.60/190,156 filed Mar. 17, 2000.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to very sensitive thermometricinstruments, known as microbolometers, which are used for the detectionand measurement of radiant energy. More specifically, the presentinvention addresses correction of microbolometer output.

2. Related Art

Infrared detectors known as microbolometers respond to impinginginfrared radiation through subtle variations in the temperature of thedetector element. The detector elements include a material having a hightemperature coefficient of resistance (TCR) such that these subtlevariations in the temperature of the detector may be sensed. The sensingmethods often employed are based on the passing of a metered electricalcurrent through the device and measuring the resulting voltage drop.Alternatively, the temperature of the detector may be sensed by biasingthe detector circuit with a known voltage and measuring the resultingcurrent. In the simplest embodiment, the microbolometer detector isconnected to a meter, and the response of the meter can be correlated tothe intensity of the impinging infrared radiation.

However, in typical applications for which an image is desired, a lensis employed to focus energy onto a two-dimensional array ofmicrobolometer detectors such that a spatially varying infrared fieldcan be detected and converted to visible imagery using electronics anddisplay means such as are commonly employed for visible imagery usingCharge-Coupled Device (CCD) cameras. The electronics typically include amultiplexing circuit in intimate contact with the microbolometer arraywhich converts the voltage or current variation of the manymicrobolometer elements to one or several multiplexed analog (e.g.,voltage variation) data streams. This analog data is then converted todigital data using an analog-to-digital converter (ADC), and is thenfurther processed to produce data for analysis or imagery on a CathodeRay Tube (CRT) or similar video monitor.

The fact that the detection means is based on the thermal variations ofthe detector causes several practical problems. First, the material mustbe thermally isolated from surrounding matter so that a sufficientlylarge (e.g., several mK) temperature variation may occur as a result ofthe weak impinging infrared energy. Liddiard, in U.S. Pat. Nos.4,574,263 and 5,369,280, and Higashi, et al., in U.S. Pat. No. 5,300,915describe a microbolometer that provides thermal isolation by depositinga semiconductor material onto a pellicle, or “micro-bridge” structurethat physically separates the detector from the supporting substrate.Second, the temperature of the supporting substrate must be stable sothat erroneous signals are not generated from its temperaturefluctuations. Experience indicates that a 15 mK variation of substratetemperature within the sampling period (or video frame rate, whicheveris greater) is acceptable, but fluctuations greater than this present asignificant source of system noise. Third, the output of themicrobolometer varies as a result of both the impinging infraredradiation, and the absolute temperature of the substrate. In this lastcase, the array output may be higher or lower at different temperatures,even if that temperature is held to within the stability requirement of15 mK. Fourth, variations in the physical construction of themicrobolometer detectors result in significant variations of the outputof individual microbolometer detectors within the array, and thesenon-uniformities must be corrected in order to obtain a low-noise image.

As a result, there exists a need for an apparatus capable of correctingthe output of a microbolometer, for example, in a focal plane array(FPA), such that the effects of thermal drift are removed or eliminated.

In the particular problem of thermal variation of the substrate,microbolometer detectors are operated at a fixed temperature, typicallywith a stability tolerance of ±0.015° C. (i.e., 15 mK). Peltier-junctionheat engines and control circuitry are commonly employed for thispurpose. While this temperature stabilization scheme works well, it isnot the ideal solution. For instance, the temperature stabilizationsystem represents a significant portion of the detector package cost.Further, it is susceptible to damage from shock or vibration, andordinarily requires tens of seconds to reach operational temperaturefrom system start-up. Also, the temperature stabilization means is amajor consumer of system power.

Since the output of the microbolometer varies as a result of impinginginfrared radiation, a number of additional noise sources and undesirableeffects occur. Referring now to FIG. 7, the microbolometer is impingedby infrared radiation from the cold shield, 608 a, the lens, 616 a, thedewar window, 606 a, as well as the signal 622 a. If the temperature ofthe cold shield 608 a, lens 616 a and dewar window 606 a remainconstant, then variation in their radiant flux also remain constant. Themicrobolometer output voltage or current due to variations in the signalemanating from source 618 a may then be determined by subtracting thefixed voltage or current offset arising from the impinging radiationfrom the cold shield, lens and dewar window. If, however, thetemperature of the cold shield 608 a, lens 616 a and dewar window 606 avary more than a few degrees Celsius, a significant source ofuncertainty in the microbolometer output, termed noise, is created.

Due to limitations in the input sensitivity range of theanalog-to-digital converter (ADC), these noise sources may cause underor over saturation of the ADC, resulting in the loss of sensitivity todesired signal data. The radiant flux from the source may also vary morethan the input sensitivity of the ADC permits, and loss of sensitivityto desired signal data may also result.

In the particular problem of modulating the detector's sensitivity towidely varying radiant flux from the source, several methods aretypically employed to maximize dynamic range. The most common method isto insert a mechanical aperture stop within the system optical path tovignette energy and effectively change the focal ratio (or “f” number).However, this method requires mechanical components that are bulky,expensive and unreliable, and require the user to manually adjust thesensitivity. A better method is to vary the duty cycle of the detectorby means of changing the time that the sensor is being sampled by thesystem electronics. This method is a distinct improvement over themanual method, but has the disadvantage of requiring a new calibrationeach time the sampling time is changed. While the calibrations can bestored in digital memory, the quantity of memory required and the numberof calibrations that must be performed tend to increase system size andcost.

In the particular problem of correcting for thermal variations of thecold shield 608 a, lens 616 a and dewar window 606 a, the most commonmethod is to perform frequent system calibrations to eliminate theseeffects. Many thermal imaging systems have built-in motorizedcalibration sources for this purpose. However, calibrating the systemfrequently reduces the availability of the system for its intended useand consumes system power. Further, the motorized calibration source(shown as 630 and 631 in FIG. 6) decreases the reliability of the systemdue to its moving parts and increases manufacturing costs.

Therefore, there exists a need for an apparatus capable of correctingthe output of a microbolometer, for example, in a focal plane array(FPA), such that the effects of thermal variations in the cold shield608 a, lens 616 a and dewar window 606 a are reduced or eliminated. Thisapparatus should also have the ability to modulate the sensitivity ofthe microbolometer to large variations in the radiant flux (signal) fromthe source so that the microbolometer output remains within the inputsensitivity range of the ADC.

SUMMARY OF THE INVENTION

It is an advantage of the present invention to provide a system andmethod for correction of microbolometer output. For example, the presentinvention provides a method to eliminate the need for gross temperaturestabilization of a microbolometer through the creation of a system thatuses electronic means to correct the temperature variation of themicrobolometer. An advantage of the present invention is that iteliminates the need for recalibration of a microbolometer appliance, forinstance a microbolometer camera, should the temperature of the focalplane array in the camera change from the temperature for which it wascalibrated. Further, rapid system readiness is possible since thermalstabilization of the focal plane array is not necessary. Specifically,this invention conditions the multiplexed output of a microbolometerfocal plane array so that the peak-to-peak voltage of the analog signalis within the range of an analog-to-digital converter's inputsensitivity at any arbitrary temperature between approximately −10° C.and 50° C.

A further general aspect of this invention is to provide a method ofcorrecting the output of a microbolometer detector system, said methodcomprising: providing a microbolometer detector system for operation inan ambient temperature range, said microbolometer detector systemfurther comprising at least one microbolometer detector, and anapparatus for reducing thermal noise; providing a temperaturestabilization system for correcting the temperature variation of themicrobolometer detector; providing an electronic system for conditioningthe output of the microbolometer detector; and applying a correctionfactor to the output signal of the microbolometer detector.

A second general aspect of the present invention is to provide a methodof correcting the output of a microbolometer detector system, saidmethod comprising: providing a microbolometer detector system foroperation in an ambient temperature range, said microbolometer detectorsystem further comprising at least one microbolometer detector, and anapparatus for reducing thermal noise; providing a temperaturestabilization system for correcting the temperature variation of themicrobolometer detector; providing an electronic system for conditioningthe output of the microbolometer detector; providing a system forvarying the responsivity of the microbolometer; applying a correctionfactor to the output signal of the microbolometer detector.

A third general aspect of the present invention is to provide anapparatus for correction of a microbolometer detector output, saidapparatus comprising: at least one microbolometer detector; a thermalnoise reduction system operationally connected to said at least onemicrobolometer detector; an electrical reference circuit connected tosaid at least one microbolometer detector; an output from saidelectrical reference circuit connected to an input of a signalconditioning circuit; and an output from said signal conditioningcircuit connected to a display device.

A fourth general aspect of this invention is to provide a method formodulating the sensitivity of a microbolometer by conditioning themultiplexed output of a microbolometer focal plane array so that thepeak-to-peak voltage of the analog signal is within the range of ananalog-to-digital converter's input sensitivity at any arbitrarysampling time.

A fifth general aspect of this invention is to provide a method forcorrecting the output of a microbolometer to reduce or eliminate theeffects of variation in the temperature of the cold shield, dewar windowand lens.

The foregoing and other features and advantages of the invention will beapparent from the following more particular description of preferredembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of this invention will be described in detail,with reference to the following figures, wherein like designationsdenote like elements, and wherein:

FIG. 1 is a schematic diagram of a detection circuit and associatedsignal processing devices in accordance with the present invention;

FIG. 2 is a graph illustrating the dynamic range of a detector output ata given temperature and dc offset;

FIG. 3 is a graph illustrating the same dynamic range of a detectoroutput at a second temperature with a second dc offset;

FIG. 4 is a graph illustrating the same dynamic range of a detectoroutput with the dc offset removed;

FIG. 5 is a graph illustrating the amplified signal of a detector outputwhen the input sensitivity of the analog-to-digital converter (ADC) ismatched to the dynamic range;

FIG. 6 is a perspective view of a detector-calibration source system;

FIG. 7 is a perspective view of a detector-calibration source systemshowing radiant flux from system components;

FIG. 8 shows a flow diagram for analog offset interpolation with thermalstabilization of the focal plane array; and

FIG. 9 shows a flow diagram for analog offset interpolation withoutthermal stabilization of the focal plane array.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although certain preferred embodiments of the present invention will beshown and described in detail, it should be understood that variouschanges and modifications may be made without departing from the scopeof the present invention. The scope of the present invention will in noway be limited to the number of constituting components, the materialsthereof, the shapes thereof, the relative arrangement thereof, etc.,which are disclosed simply as an example of the preferred embodiments.

Infrared energy focused on a two-dimensional microbolometer detectorfocal plane array (FPA) produces an output that varies from detector todetector within the plurality of detectors that form the array. Evenwhen the energy emitted by a spatially and temporally uniform object isfocused on the FPA, the several microbolometer detectors comprising thearray provide significantly different outputs. This is due to severalfactors, including the nature of the energy distribution focused on thefocal plane array by the optical system, which typically follows acos^(n)θ profile (where n is a number between 2 and 4, depending on theoptical system design), and fabrication variations from detector todetector. These two factors (optical energy variations and manufacturingtolerances) are collectively known as “fixed pattern noise.”

Fixed pattern noise can be corrected through a calibration method thatentails focusing a spatially uniform energy field on a microbolometerdetector array, determining the average response of the array, and thencalculating the amount that the output of each microbolometer variesfrom the average. This data is stored in random access memory (RAM)tables for future array output corrections. The non-uniformitycorrection values are applied at several different stages in the signalprocessing chain. An example of the signal processing chain is shown inFIG. 1.

In FIG. 1, a microbolometer detector 102 includes the temperature sensorelement or microbolometer 110, which is typically placed in a Wheatstonebridge 130 configuration biased by an electrical potential difference112, 114 of approximately 2-5 volts DC, with reference resistances104(array), 106(row), 108(column) which are either heat sunk to asubstrate (not shown), or thermally isolated from the substrate in thesame manner as the temperature sensor element 110. The column reference108 and row reference 106 provide partial correction for thermalvariations in the substrate, while the array reference 104 (which is notilluminated) provides partial correction for the temperature-varyingthermal characteristics of the microbolometer's thermal isolationstructure. However, since the reference resistances 104, 106, 108 do nothave precisely the same resistance-temperature relationship as thetemperature sensor element 110, this correction is not complete. Thoseskilled in the art will recognize that several different possibledetector readout architectures have been demonstrated, such as thosedescribed by Parrish, et al., in U.S. Pat. No. 5,756,999, and that thepresent invention may be adapted to any of a number of analog detectoroutput architectures.

In the example architecture, the analog output of the Wheatstone bridge130 is input into an analog-to-digital converter (ADC) 122 fordigitization of the analog signal. However, since the temperature sensorelement 110 output varies as a function of temperature, this signal mustbe conditioned by a signal conditioning circuit, which includesamplifier stages 116, 118. The function of the amplifier stages 116, 118is to make the full dynamic signal variation available to the ADC 122.This is because the input sensitivity of the ADC 122 is not well matchedto the dynamic variation of the temperature sensor element 110 output;and because the temperature sensor element 110 output has a significant“pedestal,” or DC offset, which is not signal information; and becausethis “pedestal” or DC offset is not fixed from detector to detector. Asan example, FIGS. 2 and 3 show a sample microbolometer detector voltageas a function of time (alternatively, this may be viewed as a timemultiplexed output of a number of different microbolometer detectors).Note that the dynamic range of the detector is a much smaller voltagepotential difference than the absolute voltage output of the biaseddetector, and that the pedestal voltage forms the difference between theoutput dynamic range and the peak output voltage.

FIG. 1 further shows that the signal conditioning employed includes adifferential amplifier 116 that removes the signal pedestal. This isshown graphically in FIG. 4. The differential amplifier 116 of FIG. 1uses a digital input value that indicates the value of the pedestal tobe removed. This may vary for each detector of an array, and varies as afunction of temperature. The fixed amplifier 118 of FIG. 1 conditionsthe output of the differential amplifier 116 such that the signal iswithin the input sensitivity of the ADC 122. The exemplary signal ofFIGS. 2-4 is shown in FIG. 5, showing that the dynamic range is nowamplified to meet the input sensitivity of the ADC 122.

Following the digitization of the signal, the digital signal is thenprocessed to remove fine non-uniform variations in the output. This isdone with the use of digital coefficient memory 124, which provides acorrection value corresponding to the difference between the output ofan individual detector in the array and the array mean when viewing aflat-field radiation source (such as a blackbody calibration source).

Another correction stage applies a gain value to the signal thatcorrects the non-uniform response of different detectors in the array tovariations in the impinging infrared energy. Together, these correctionvalues remove system fixed pattern noise from the image. However, aswill be shown, the correction values are only serviceable for thetemperature at which the system was calibrated. The responsivity, , ofthe microbolometer is defined as: $\begin{matrix}{\Re = \frac{{BV}\quad \alpha \quad ɛ}{G\left( {1 + {4\pi \quad f^{2}\tau^{2}}} \right)}} & (1)\end{matrix}$

where:

B is the bridge factor R_(L)/R_(L)+R R_(L) is the load resistance

R is the detector resistance

V is the detector bias voltage

α is the temperature coefficient of resistance (TCR) of the material

ε is the emissivity of the microbolometer

G is the thermal conductance

τ is the thermal time constant, C/G

C is the thermal capacitance

f is the frequency (frame rate)

The parameter of interest is α, the Temperature Coefficient ofResistance (TCR), as it is the determining factor of the temperaturevariation of the output of the detector if all other factors are heldconstant. Semiconductors are typically chosen for these types ofmicrobolometer detectors since they possess high TCR values ofapproximately 2.5%/° C. The disadvantages of too high a TCR are higher(1/f) noise, and variance of the TCR with respect to temperature.Semiconductors possess strong negative nonlinear TCR characteristics.The resistivity of the material is derived from the amount of freecharge carriers within the substance, wherein the quantity of thesemobile carriers increases with increasing temperature. However, themobility of these carriers varies inversely to temperature by providinga generally gradual increase in mobile carriers. Thus, the TCR of amaterial is based on the activation (bandgap) energies of the electrons(the mobile carriers in this case) and is described by: $\begin{matrix}{\alpha = {- \frac{\Delta \quad E}{{KT}^{2}}}} & (2)\end{matrix}$

where:

ΔE is one-half the bandgap energy

K is Boltzmann's constant

T is the absolute temperature (° K.)

As described above, the TCR of a material varies with temperature.Therefore, the response of a detector is also a function of the detectortemperature, even when viewing the same object. This variation causesthe output of the detector to increase or decrease such that iteventually saturates the ADC 122.

Another important implication of equation (1) is that the detectorresponse is a function of several parameters for which manufacturingtolerances create significant variations from detector to detector.Specifically, the thermal conductance, G, and the thermal capacitance,C, may vary significantly, particularly when the microbolometer isfabricated using micro-machining techniques. These manufacturingtolerances are a significant source of the microbolometer arraynon-uniform response, termed fixed pattern noise.

Correction of output where the activation energy is nearly constant withrespect to temperature will now be addressed. Certain semiconductormaterials have activation (bandgap) energy values which are nearlyconstant with temperature in the temperature region of interest(approximately −10° C. to +50° C.). A noteworthy example of such amaterial is amorphous silicon, particularly H:α-Si.

One goal of the present invention is to define a corrective gain, M, tomultiply by the analog offset value that was established during thecalibration process. The gain M is a function of T₁, being the focalplane array (FPA) temperature that is sensed by the temperature sensor,and corresponds to the addition of change in the TCR at a referencetemperature α₀(T₀). Hence: $\begin{matrix}{{\alpha_{0}\left( T_{0} \right)} = {- \frac{\Delta \quad E}{{KT}_{0}^{2}}}} & (3)\end{matrix}$

 Δα=α₁−α₀   (4)

 M(T ₁)α₀=α₀+Δα  (5)

Combining equations (4) and (5): $\begin{matrix}{{M\left( T_{1} \right)} = \frac{\alpha_{1}}{\alpha_{0}}} & (6)\end{matrix}$

Substituting equation (3): $\begin{matrix}{{M\left( T_{1} \right)} = \frac{- \frac{\Delta \quad E}{{KT}_{1}^{2}}}{- \frac{\Delta \quad E}{{KT}_{0}^{2}}}} & (7)\end{matrix}$

Assuming activation energies are constant with temperature:$\begin{matrix}{{M\left( T_{1} \right)} = {\frac{T_{0}^{2}}{T_{1}^{2}} = \left( \frac{T_{0}}{T_{1}} \right)^{2}}} & (8)\end{matrix}$

Those skilled in the art will recognize that similar predictablevariations in the resistance of the detector material, as a function oftemperature, may be identified for non-semiconductor materials such asmetals (including e.g., platinum) or organic materials such as proteins.While the physical processes within these alternative materials differfrom the physical processes within semiconductors, a similar gain may bedefined which may then be employed to correct the temperature variationsin the array output.

Microbolometer output may deviate from the ideal temperature function asexpressed in the simple gain value shown in equation (8). For instance,the foregoing analysis assumes that the thermal capacitance, C, and thethermal conductance, G, are not strong functions of temperature.However, this is not a valid assumption over large temperature ranges(tens of kelvin). A theoretical analysis based on the actualtemperature-dependant thermal capacitance, C, and thermal conductance,G, functions will result in a polynomial expression for the correctivegain, M. In this general case, a second method as described below may beemployed.

Correction of microbolometer output for the alternative case whereactivation energy is a function of temperature is now presented. Certainsemiconductor materials do not present a linear response withtemperature due to variations in the bandgap energy, as well as otherfactors. These other factors may include variation in the thermalconductivity or thermal capacitance of the microbolometer isolationstructure; variations in surface emissivity with temperature; orvariations in the behavior of the multiplexing integrated circuitelements as a function of temperature. In these cases, a more generalcorrection scheme must be employed to remove the pedestal from theanalog microbolometer output. Since the key parameters, thermal timeconstant, τ, and temperature coefficient of resistance, α, are functionsof temperature following equations (9), the responsivity equation (1)becomes a function of temperature having polynomial terms. The function,=ƒ(T) may be derived expediently from empirical data.

In addition to temperature variations in the responsivity equation dueto material properties such as activation energy, thermal conductivity,G, or enthalpy, the thermal time constant, τ, is a function ofmicrobolometer geometry and therefore manufacturing tolerances. Thealgorithm for calculation of the pedestal removal function must includemicrobolometer-specific terms to account for these manufacturingtolerances, namely:

C=ƒ(T),G=ƒ(T)∴τ=ƒ(T),α=ƒ(T)∴=ƒ(T)  (9)

The calibration of the microbolometer array 610 (FIG. 6) is accomplishedthrough a process whereby the temperature sensor element 110 (FIG. 1) isexposed to a uniform source of focused infrared energy 622, and theoutput 132 of each microbolometer 110 is sequentially sensed.

FIG. 7 shows components of the microbolometer-calibration source system600 a. A focal plane array 610 a is mounted on a multiplexing integratedcircuit 612 a which is in turn mounted on a Peltier-junction heat engine604 a which provides a temperature control means. The focal plane array610 a is shielded from undesired infrared energy, emitted by vacuumdewar 602 a through the use of a cold shield 608 a. The vacuum dewar 602a provides thermal isolation to the internal contents, effectivelyeliminating convection heat transfer within the enclosed volume. Thevacuum dewar 602 a includes an IR transmissive vacuum window 606 a sothat the desired radiation from the IR blackbody radiation source 618 a,housed in 620 a, may emit IR radiation 622 a that in turn will impingeon the focal plane array 610 a. An IR refractive lens element 616 afocuses the IR radiation 622 a on the focal plane array 610 a. The focalplane array 610 a is composed of a plurality of individualmicrobolometer detectors 102 (FIG. 1), which are preferably arranged ina rectilinear array. A plurality of electrical interconnects 614 aprovide an electrical connection between the internal vacuum space andthe external environment. A temperature sensor 624 a is mounted inintimate contact with the multiplexing integrated circuit 612 a and/orthe focal plane array 610 a such that the temperature of the focal planearray 610 a may be sensed externally to the vacuum dewar 602 a. Thetemperature sensor 624 a may be of the thermister type or the diodetype. A typical diode variety is known to those skilled in the art asmodel 2n2222.

The multiplexing integrated circuit 612 is also known as a Read-OutIntegrated Circuit (ROIC), and it is typically fabricated as anintegrated circuit within the substrate. The function of the ROIC is toprovide electrical connections between the temperature sensor element110 and the reference resistors 104, 106, 108, as shown in FIG. 1, ifany are provided. In addition, the ROIC provides bias voltages, whichare preferably pulsed to avoid excessive parasitic heating of themicrobolometer detector 102. The ROIC further time division multiplexesthe output 132 of each microbolometer detector 102, so that eachmicrobolometer detector 102 in the array 610 is biased in sequence, andis output from the focal plane array 610 to one or more electricalinterconnects 614 for processing by external circuitry.

The external circuitry provides conditioning of the analog output asdescribed herein, and digitizes the output 132 using ananalog-to-digital converter 122 (FIG. 1). The digital output 138 foreach microbolometer detector 102 of the array 610 is stored in randomaccess memory 124 for manipulation and processing by a microprocessor126, using a predetermined instruction code.

The detection circuit and signal processing device system 100 (FIG. 1)is calibrated by thermally stabilizing the focal plane array 610 at atemperature T1 corresponding to the lowest desirable operatingtemperature using the Peltier-junction heat engine 604 and thetemperature sensor 624 for control feedback. Referring now to FIG. 1,the output 132 of the focal plane array microbolometer detector element102 is input to a low noise differential amplifier 116, the controlinput 134 of which is attached to a digital-to-analog converter (DAC)120, such that when a digital control signal is input to the DAC 120, ananalog voltage is produced which is then input to the differentialamplifier 116. The differential amplifier 116 amplifies the potentialdifference that is applied to its input terminals. Therefore, the analogcontrol input 134 effectively provides a subtraction value that maysubtract some or all of the signal voltage. The output 136 of thedifferential amplifier 116 is amplified by a fixed amplifier 118, whichmay be incorporated into the differential amplifier 116, or may be adiscrete device. The gain of the amplifiers 116, 118 is selected so thatthe dynamic range of the microbolometer detector 102 signal is matchedto the input sensitivity of the ADC 122. A typical value of the inputsensitivity of an ADC 122 is 0-2 volts dc, whereas the typical output132 of a focal plane array microbolometer detector 102 is 0-5 volts dc.Therefore, the differential amplifier 116 must provide a minimumsubtraction voltage of the difference between these two values. If thesubtraction voltage is incorrect, the ADC 122 will be saturated. Acorrect value of the subtraction voltage will cause the ADC 122 toproduce an output 138 which is approximately in the center of itsdigital range. During calibration, the value of the subtraction voltageis determined by arbitrarily selecting a voltage, and then determiningif the focal plane array detector element output 132 causes the ADC 122digital output to be in the center of its range. The process isperformed one or more times until the correct subtraction voltage hasbeen determined for each microbolometer detector 102 of the focal planearray 610. The subtraction voltage values so selected are stored so thatthey may be used to condition the future output of the focal plane array610. These data are termed analog offset correction values. Since eachmicrobolometer detector 102 has an independently selected analog offsetvalue when viewing a uniform radiation field, the values remove both themicrobolometer detector output pedestal, and the spatial fixed patternnoise from the array image.

The above process is repeated at additional temperatures T2, T3 . . .T_(n), such that a number of analog offset data sets are produced.Preferably, at least one data set is produced for each 1 degree segmentof the desired operating range. A curve fit of the analog offset data isperformed for the several temperature values so recorded. A curvefitting method is applied to the data so that an equation is derivedwhich produces a good fit with the analytical data. Curve fittingmethods such as the least squares method, or any of a number ofnumerical or graphical methods known to those skilled in the numericalanalysis art may be used. The empirical equation is preferably in theform of:

offset=ƒ(T)=AT ³ +BT ² +CT+D   (10)

where A, B, C, or D are constants associated with the unique data set. Tis the temperature for which the offset is desired.

An exemplary analog offset correction method is described next. Thetemperature sensor 139 produces an analog voltage output (such as thedepicted diode) or a voltage drop (such as a thermister) when biasedwith a current from current source 141. This voltage is input to ananalog-to-digital converter 140 which sends its output to signalprocessing circuitry or a microprocessor 126 (FIG. 1) which isprogrammed to apply equation (10) to the temperature value. The outputof this equation is then input to the digital-to-analog converter 120which in turn produces an analog output 134 to the differentialamplifier 116, causing the subtraction voltage to be applied to thefocal plane array output 132.

To increase the speed of calculation, all offset values for the arraymay be averaged for each temperature value (T1, T2, etc.), and oneaverage function for the entire array may be computed. However, in orderto remove fixed pattern noise from the output, the deviations in theoutput from microbolometer detector to microbolometer detector must beaccounted for. Therefore, a temperature T_(n) is selected which isapproximately in the center of the operating temperature range. Thepreviously described calibration method is applied at this temperatureto obtain a data set which contains a baseline value of the analogoffset for each microbolometer detector. This average function isapplied to the analog offset values as a gain defined as the ratio ofthe curve fit of the offset function for any arbitrary temperature T,such that T is a temperature within the calibration temperature range;plus a constant K; to the offset average for the mid-range baselinetemperature (a constant) plus a constant K, where K is an empiricallydetermined value which is the difference between the digital equivalent(in digital counts) of the minimum offset voltage range and systemground voltage. The constant K has the effect of making the ratio ofanalog offset values an absolute ratio of a digital representation ofvoltages. $\begin{matrix}{{M(T)} = \frac{{offset}_{average}\left( T_{n} \right)}{{offset}_{average}(T)}} & (11)\end{matrix}$

This correction factor is applied to the microbolometerdetector-specific offset value for temperature T_(n) for themicrobolometer detector (pixel), such that the new offset value for theindividual detector element is as follows:

offset_(corrected)(T)=offset_(pixel)(T _(n))×M(T)   (12)

Note that each element in the array will have an individually calculatedanalog offset value. If a microbolometer focal plane array does not havea sufficiently uniform output (such as +/−2% of the array mean responsewhen viewing a uniform source of infrared energy), the foregoing methodmay not provide sufficient correcting of fixed pattern noise. In thesecases, a second method is used as follows: The corrected offset at anyarbitrary temperature T may be alternatively determined by selecting abaseline temperature T_(n) which is preferably in the center of theoperational temperature range, and then by computing the differencebetween the offset value for each detector at this temperature and theoffset value for each detector at any other temperature, T, within theoperational temperature range. Thus we can define a new function, theoffset difference function:

offset_difference (T)=offset(T _(n))−off-set(T)

To improve the speed of calculation, offset values corresponding to manydifferent detectors may be averaged to create an average offsetdifference function vs. detector temperature. This function ispreferably in the form of a polynomial expression that is determinedthrough the process of empirical curve fitting of calibration data. Thefixed pattern noise may then be removed by algebraically adding theoffset difference function so created from pixel offset values to obtaina new offset value for each detector element as follows:

offset_(corrected)(T)=offset_(pixel) (T _(n))+offset_difference (T)

For example, consider a detector that is exposed to a calibration sourceof infrared energy and further that the detector substrate is maintainedat the calibration temperature, T_(n)=20° C. When this example detectoris sampled by the sample and hold circuitry, the resulting voltage is2.5 volts. If the desired mean input voltage sensitivity of the ADC is2.0 volts, this detector is said to have an offset of 0.5 volts, or2.5−2.0=0.5 volts. This value (i.e., 0.5 volts) is equivalent to theoffset_(pixel)(T_(n)) in the foregoing equation. If the substratetemperature of this example detector is then changed to 10° C., theresulting voltage would be 3.0 volts owing to the change in the TCR,(equation 2), and its impact on the responsivity as shown in equation 1.Thus, if a new calibration is performed at temperature T (i.e., 10° C.),a new offset voltage would be selected. This new offset voltage is3.0−2.0=1.0 volts. Thus, the offset difference function would result ina value of 0.5 volts for an input value of 10° C. For example:

offset_(corrected)(10° C.)=offset_(pixel)(10° C.)+offset_difference(10°C.)=0.5 volts=0.5 volts=1.0 volts

For the previous example of one detector, this approach is quitestraightforward and may be calculated readily. However, for a focalplane array of many thousands of detectors, the computation task becomesquite difficult. To speed the calculation, the offset differencefunction may be calculated based on the average of many differentdetectors. The offset_(pixel) term would continue to have the unique,detector specific calibration offset value. For speed of calculation,the per pixel offsets, offset_(pixel) (T_(n)), may be digitized using ananalog to digital converter and stored in random access memory. Thus,the system will have one offset difference equation and many per pixeloffset values. This method has the benefit of combining detector elementunique calibration data with array average data, and does so in a mannerwhich permits rapid calculation.

Still, the foregoing method does introduce error. For instance, if thearray average offset difference function resolved to 0.4 volts at 10°C., the detector in the previous example would be assigned an offsetvalue of 0.5+0.4=0.9 volts instead of the required 1.0 volts. Thus, anadditional improvement is needed to correct for pixel variances. Anadditional correction factor may be employed for this purpose. Thecorrection factor is determined by taking the ratio of the offsetdifference function based on the array average data (offset_difference)and the actual difference between the pixel offset at the calibrationtemperature, T_(n), and the pixel offset at temperature T. Note that inorder to determine the offset correction factor, two calibrations mustbe performed, one at T_(n) and one at T corresponding respectively tothe calibration temperature and a second arbitrary temperature which ispreferably approximately 10° C. different from the first. For example,consider that for the pixel in the previous example, the twocalibrations yield output voltages of 3.0 volts at T and 2.5 volts atT_(n). Thus, if the center of the input sensitivity range is 2.0 volts,the offset for temperature T would be 1.0 volts and the offset for T_(n)would be 0.5 volts. If, however, the array had an average offsetdifference function that resolved to 0.4 volts at T, an error wouldresult as follows:

offset_(corrected)(T)=offset_(pixel)(T)+offset_difference(T) =0.5volts+0.4 volts=0.9 volts

But, as shown previously, the corrected offset voltage is 1.0 volts, anerror of 0.1 volts. The correction factor for the offset difference isthen equal to the ratio between the actual offset voltages determined bycalibration at temperatures T and T_(n) for each element and the arraymean offset difference function:

correction_factor=[offset_(corrected)(T)−offset_(pixel)(T_(n))]/offset_difference(T)

For the previous example, this resolves to:

correction_factor=[1.0 volts−0.5 volts]/0.4 volts=0.5 volts/0.4volts=1.25

The correction factor is applied as follows:

offset_(corrected)(T)=offset_(pixel)(T_(n))+offset_difference(T)*correction_factor

To illustrate the application of this function to the example detectorelement at temperature T:

offset_(corrected)(T)=offset_(pixel)(T_(n))+offset_difference(T)*correction_factor=0.5 volts=0.4 volts*0.25=1.0 volts

One correction factor and one baseline calibration value(offset_(pixel)) is used for each pixel in the focal plane array for alloperational temperatures; one offset difference function is used for theentire array to describe the variation in offset as a function oftemperature. Thus, a combination of per pixel calibration data (both anoffset and a gain) and a mean polynomial function is used to provide anaccurate means of calculating the required analog offset at anytemperature and for any detector element in an array. The advantage ofthis system over the simple offset function of equation (10) is that foreach frame of data comprising tens of thousands of pixels read inapproximately {fraction (1/60)}th of a second, only one polynomialfunction need be calculated, all other calculations are simplemultiplication (the gain or correction factor term) and a simpleaddition (the baseline calibration offset) need occur. This permits theapplication of an offset function on a real time basis to analog videofrom the focal plane array.

Rapid readiness of the detection and correction technique from systemstart-up will result from application of the correction method discussedherein. It is another advantage of the present invention to minimize thestart-up time of the microbolometer detector system by one of twomethods: temperature stabilization at the initial temperature; orelimination of the temperature stabilization requirement.

The foregoing calculation method for offset permits the application ofadditional terms to address the other objects of the invention. It is anadvantage of the present invention that the microbolometer detectoroutput may be easily modulated to compensate for large deviations in themagnitude of impinging radiation. Referring now to equation (1), thedetector responsivity may be varied by changing the voltage V, orsampling frequency ƒ. Alternatively, the responsivity may bemechanically varied through vignetting. Each of these methods requires adifferent analog offset value in order to assure that the detectoroutput is well matched to the ADC input sensitivity and hence not over-or undersaturated. For instance, when viewing a hot object such as aflame, the output of the detector increases such that saturation of theADC may occur. In these cases, it is desirable to reduce the sensitivityof the microbolometer array through either reduction in the time thateach detector element is sampled using the sample and hold circuit, orreduction of the bias voltage, or increase in the applied analog offset,or any combination of these three methods. Since the system forcompensating for thermal fluctuations of the detector permits real timecalculation of analog offsets, we may calculate a new offset value tocompensate for adjustments to the input sensitivity. The best way to dothis is to perform a series of calibrations, parametrically adjustingthe sampling time, bias voltage or degree of mechanical vignetting,recording the resulting analog offset values which satisfy therequirements that the output of the microbolometer be well matched tothe input sensitivity range of the ADC, and curve fitting the results.Following this process, an expression may be created in a polynomialform which has input parameters of sampling time, voltage and/or degreeof mechanical vignetting and which outputs the offset differencefunction. Thus, the new equation for offset is:

offset_(corrected)(T)=offset_(pixel)(Tn)+offset_difference(T,ƒ,V,mechanical vignetting)*correction_factor

The analog offset may be applied in this manner, or may be applied as anadditional correction factor to the entire right side of the offsetfunction:

offset_(corrected)(T)=correction_factor²(T,ƒ,V, mechanicalvignetting)*[offset_(pixel () Tn=offset₁₃difference(T)*correction_factor]

Large deviations in the temperature of system components such as thelens, dewar window, cold shield, or other components which are within“view” of the detectors give rise to deviations in the output of themicrobolometer elements, and may cause the ADC to be under- oroversaturated. In the present invention, temperature sensors are placedon these components to record their temperature. This temperatureinformation is used in conjunction with offset calibrations to correlatecomponent temperature with the required analog offset voltage necessaryto bring the microbolometer output to the center of the inputsensitivity range of the system ADC. This correlation is curve fit suchthat a series of mathematical functions of analog offset voltage vs.component temperature are created. These functions may be applied to theanalog offset function above to provide additional correction utility tothe offset function. An example function is presented:

offset_(corrected)(T)=offset_(pixel)(Tn)+offset_difference(T,ƒ,V,mechanical vignetting, T _(lens) , T _(coldshield) , T_(dewar window))*correction_factor

Referring to FIG. 1, the hardware used in these methods is brieflydescribed. The temperature value, which is digitized using theanalog-to-digital converter (ADC) 140, is input to a microprocessor 126which has random access memory 124 connected to it using a digital databus. The microprocessor 126 reads the calibration data from the randomaccess memory 124 and applies the digital temperature value to equations(11) and (12), producing a new corrected offset value data set for themicrobolometer detector 102. This is stored in random access memory 124and is applied to correct the analog output 132 of the microbolometerdetector 102.

The correction method employing temperature stabilization at the initialtemperature is presented first. Referring now to FIG. 8, a process flowscheme 750 is shown for the initial start-up condition wherein thedesire is to apply heating or cooling to the focal plane array (FPA) tomeet some predetermined operation temperature.

Prior to use, the focal plane array is calibrated such that an analogoffset data set and curve-fit equation are stored in memory.

Upon system power-up 700, several functions are performed. In additionto the necessary power supply stabilizations 701, microprocessor boot-uproutines 703, 704 etc., the microbolometer FPA is heated or cooled 702to bring it to a specific predetermined temperature corresponding to itscalibration data set. The thermal stabilization may be performed, e.g.,using a Peltier-junction heat engine, which performs this function inapproximately 10-60 seconds, depending on the difference between theinitial temperature and the desired temperature. Alternatively, the FPAtemperature is sensed 706 at power-up, and the temperature control meansstabilizes the FPA at whatever arbitrary temperature that the FPAhappens to be.

Simultaneous to the thermal stabilization 705, the temperature sensor isread 706 by the computer and an algorithm is applied to correct 708 theanalog output of the FPA to bring its peak-to-peak voltage output intothe sensitivity range of the analog-to-digital converter (ADC) 122 (FIG.1). The predetermined per-pixel stored offset data 707 is used in thecorrection algorithm 708 to calculate analog offset values which arestored in memory 709.

As the first analog frames are produced 710 by the focal plane array(FPA), the previously calculated offset values are applied 711 to theseframes. The resulting signal is then converted to digital data 712, andwhen used, the stored digital offset values are applied 713. Finally,the previously stored gain values are applied 714, yielding thecorrected images 715.

The system depicted in FIG. 8 presumes that the compensation scheme isapplied only at start-up. This system requires continuous stabilizationof the FPA temperature, thus consuming system power at a greater ratethan an unstabilized system.

An alternative approach, namely elimination of the temperaturestabilization requirement, or passive temperature stabilization is nowdiscussed. In order to eliminate the temperature stabilization device(for example, the Peltier-junction heat engine), the circuit board ismounted on a relatively large thermal mass, such as a block of copper,having a mass of approximately 10 grams or more. The thermal massabsorbs energy and decreases the slope of the temperature/time curve asenergy is input to the focal plane array 610 (FIG. 6). The temperatureof the focal plane array 610 is sensed, and the offset correction schemeis applied to the stored data sets, as previously described. However, asthe focal plane array 610 changes temperature, new calibration data setsare calculated and applied to the focal plane array's individualdetector analog output 132 (FIG. 1). This process is repeated wheneverthe temperature of the array 610 deviates by a predetermined increment,such as 0.015° C., or as much as 1° C. To minimize the impact of thetemperature excursions of the focal plane array 610, a process know tothose skilled in the art as automatic gain control is applied to thedigital output 138 of the ADC 122 (FIG. 1). This effectively eliminatesnoise for temperature excursions as great as 1° C.

FIG. 9 describes a system wherein the FPA temperature is allowed todrift (i.e., it is not stabilized), and repeated temperaturecompensation adjustments of the FPA output are performed to keep the FPAoutput within the range of the ADC. While this system doesn't requirestabilization, it does require additional computing capacity. Thus, adesign trade-off study must be performed to determine which method isappropriate for a given application.

Referring now to FIG. 9, the process flow scheme 850 is shown for theinitial start-up condition wherein the desire is to avoid heating orcooling the focal plane array (FPA) to meet some predetermined operationtemperature.

Prior to use, the focal plane array is calibrated such that an analogoffset data set and curve-fit equation are stored in memory. Upon systempower-up 800, several functions are again performed. The necessary powersupply stabilizations 801, and microprocessor boot-up routines 802, 803etc., are performed.

In this case, as the first analog frames are produced 804 by the focalplane array, the temperature is sensed and correction algorithms arecalculated 806. At this time, the decision 813 whether or notrecalibration is required is made based on whether the sensedtemperature 814, has deviated by a predetermined amount from somenominal value. If recalibration is required, the correction algorithmsare recalculated at 806.

The predetermined per-pixel stored offset data 805 are used in thecorrection algorithm 807 to calculate the analog offset values which arestored 808 in memory.

The resulting signal is then converted 809 to digital data, and thepreviously stored gain values are applied 811, yielding the correctedimages 812.

In the event that the sensed 814 focal plane array temperature does notchange significantly, recalibration 813 will not be required, and thedigital offset 810 applied to the data will remain unchanged.

However, if the focal plane array changes temperature, these changes aredetected 814 and recalibration 813 of the data sets and correctionalgorithms is performed. This updated information is then used to applya different analog offset 808 to the signal. This process is repeatedwhenever the temperature of the array deviates by a predeterminedincrement.

Further refinements to this system include the addition of a cold shield608 (FIG. 7) so that variations in the temperature of the container thathouses the FPA 610 do not perturb the validity of the calibrationvalues.

Another improvement to the system includes the addition of a moveablecalibration source 630 mounted on a servo motor or other actuator 631.The moveable calibration source emits spatially uniform infrared energyand is used when image non-uniformity (termed fixed pattern noise), isto be removed from the image signal. This noise will typically includethermal noise from the change in temperature of the cold shield 608,dewar 602 or vacuum window 606, all of which is not completely correctedfor through the analog offset correction algorithm scheme. It is used asfollows: The calibration source 630 is placed so as to block theimpinging energy 622, and such that the impinging energy 632 from thecalibration source illuminates the focal plane array. In this state thedigitized output of each detector element in the focal plane array 610is compared to the average output of all array elements. A digitalsubtraction or addition value is calculated such that the output of thearray is uniform once the correction value is applied. The calibrationsource 630 is removed when the process is over so that the scene thermalenergy 622 again impinges the detector. This process may be repeated asneeded to maintain a low-noise image.

Following digitization of the data, the data is output to a videoprocessor 128 (FIG. 1) which converts the data into a usable form, suchas output to a machine vision system in a conditioned digital form, oroutput to a monitor such as a liquid crystal display (LCD) or cathoderay tube (CRT), for viewing by the system user. The entire array ispreferably output sequentially at a frame rate in excess of 15 framesper second if smooth imaging of movement is desired. Alternatively, thearray may be readout only one time for still imagery, or at some otherframe rate as required by the need for thermal imaging data.

While this invention has been described in conjunction with the specificembodiments outlined above, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, the preferred embodiments of the invention as setforth above are intended to be illustrative, not limiting. Variouschanges may be made without departing from the spirit and scope of theinvention as defined in the following claims.

We claim:
 1. A method of correcting the output of a microbolometer detector system, said method comprising: providing a microbolometer detector system for operation in an ambient temperature range, said microbolometer detector system further comprising at least one microbolometer detector, and an apparatus for reducing thermal noise; providing a temperature stabilization system for correcting the temperature variation of the microbolometer detector; providing an electronic system for conditioning the output of the microbolometer detector wherein the electronic system: responds to variations in a radiant flux signal from a source; modulates the sensitivity of the microbolometer detector to the variations in the radiant flux signals; converts the output from the microbolometer detector from an analog signal to a digital signal using an analog-to-digital converter; and limits the output from the microbolometer detector so that the microbolometer detector output remains within an input sensitivity range of the analog-to-digital converter; and applying a correction factor to the output signal of the microbolometer detector.
 2. The method of claim 1, wherein the ambient temperature range is between approximately −10° C. and +50° C.
 3. The method of claim 1, wherein the apparatus for reducing thermal noise is selected from the group consisting of a lens, a dewar window, and a cold shield.
 4. A method of correcting the output of a microbolometer detector system, said method comprising: providing a microbolometer detector system for operation in an ambient temperature range, said microbolometer detector system further comprising at least one microbolometer detector, and an apparatus for reducing thermal noise; said apparatus for reducing thermal noise being selected from the group consisting of a lens, a dewar window, and a cold shield; providing a temperature stabilization system for correcting the temperature variation of the microbolometer detector; providing an electronic system for conditioning the output of the microbolometer detector; and applying a correction factor to the output signal of the microbolometer detector wherein applying the correction factor to the output signal of the microbolometer detector decreases effects of temperature variations in the lens, dewar window, or cold shield.
 5. A method of correcting the output of a microbolometer detector system, said method comprising: providing a microbolometer detector system for operation in an ambient temperature range, said microbolometer detector system further comprising at least one microbolometer detector, and an apparatus for reducing thermal noise; providing a temperature stabilization system for correcting the temperature variation of the microbolometer detector; providing an electronic system for conditioning the output of the microbolometer detector wherein the electronic system performs an algorithm to calculate a corrected offset value based on the difference between an individual microbolometer detector temperature and a baseline temperature; and applying a correction factor to the output signal of the microbolometer detector.
 6. The method of claim 5, wherein the baseline temperature is approximately at the center of the operational temperature range of the microbolometer detector.
 7. The method of claim 5, wherein an average offset difference function vs. detector temperature is calculated from a plurality of offset values corresponding to different microbolometer detectors.
 8. The method of claim 7, wherein the average offset difference function is a polynomial expression.
 9. The method of claim 8, wherein the average offset difference function is a polynomial expression determined via empirical curve fitting of calibration data for the plurality of microbolometer detectors.
 10. The method of claim 9, wherein thermal noise is substantially removed by calculating a new offset value, said new offset value the result of algebraically combining the offset difference function with a pixel offset value for each microbolometer detector.
 11. The method of claim 10, wherein the offset difference function is calculated based on the average of a plurality of different microbolometer detectors.
 12. The method of claim 11, wherein the pixel offset values are stored in a memory.
 13. The method of claim 9, wherein a pixel variance correction factor is determined by calculating the ratio of the offset difference function, based on microbolometer detector data, and an actual difference between the pixel offset at a calibration temperature and the pixel offset at a second temperature.
 14. The method of claim 13, wherein the pixel variance correction factor is applied to the plurality of microbolometer detectors on a real time basis.
 15. A method of correcting the output of a microbolometer detector system, said method comprising: providing a microbolometer detector system for operation in an ambient temperature range, said microbolometer detector system further comprising at least one microbolometer detector, and an apparatus for reducing thermal noise; providing a temperature stabilization system for correcting the temperature variation of the microbolometer detector; providing an electronic system for conditioning the output of the microbolometer detector; providing a system for varying the responsivity of the microbolometer; applying a correction factor to the output signal of the microbolometer detector.
 16. The method of claim 15, wherein the apparatus for reducing thermal noise is selected from the group of system components consisting of a lens, a dewar window, and a cold shield.
 17. The method of claim 15, wherein varying the method of varying the responsivity is selected from the group consisting of: varying the applied bias voltage, varying the sampling frequency, or varying the degree of mechanical vignetting applied to the microbolometer detector.
 18. The method of claim 17, wherein an offset value is calculated by performing a series of calibration steps: adjusting the sampling time, bias voltage, or degree of mechanical vignetting; recording the resulting analog offset values which satisfy the requirement that the output of the microbolometer detector be well matched to the input sensitivity range of a analog-to-digital converter; and curve fitting the results.
 19. The method of claim 18, further comprising the step of applying an analog offset correction factor.
 20. The method of claim 18, further compensating for variations in the temperature of system components, said steps comprising: providing temperature sensors operationally connected to said system components; monitoring said temperature sensors; correlating system component temperatures with an analog offset voltage required to bring said microbolometer detector output to the approximate center of the input sensitivity range of the analog-to-digital converter; curve fitting the correlated system component temperatures and the analog offset voltages; and producing at least one mathematical function of analog offset voltage vs. system component temperature.
 21. An apparatus for correction of a microbolometer detector output, said apparatus comprising: at least one microbolometer detector; a thermal noise reduction system operationally connected to said at least one microbolometer detector; an electrical reference circuit connected to said at least one microbolometer detector; an output from said electrical reference circuit connected to an input of a signal conditioning circuit wherein the signal conditioning circuit further comprises: a differential amplifier; a first reference source; a signal processor; an output from said electrical reference circuit connected to a first input of said differential amplifier; an output from said first reference source connected to a second input of said differential amplifier; and an output from said signal processor connected to said first reference source, wherein the output of the first reference source is determined by the signal processor; and an output from said signal conditioning circuit connected to a display device.
 22. The apparatus of claim 21, further including: a temperature sensor operationally connected to said microbolometer; a second reference source operationally connected to said temperature sensor; an analog-to-digital converter operationally connected to said temperature sensor, said second reference source, and to said signal processor.
 23. The apparatus of claim 21, wherein the first reference source is a voltage source.
 24. The apparatus of claim 23, wherein the voltage source is a digital-to-analog converter.
 25. An apparatus for correction of a microbolometer detector output, said apparatus comprising: at least one microbolometer detector; a thermal noise reduction system operationally connected to said at least one microbolometer detector; an electrical reference circuit connected to said at least one microbolometer detector; an output from said electrical reference circuit connected to an input of a signal conditioning circuit wherein the signal conditioning circuit contains an algorithm to calculate a corrected offset value based on the difference between an individual microbolometer detector temperature and a baseline temperature; and an output from said signal conditioning circuit connected to a display device.
 26. The apparatus of claim 25, wherein the baseline temperature is approximately at the center of the operational temperature range of the microbolometer detector. 